The invention presented herein relates to apparatus for determining the validity of paper securities, currency, documents, and the like. The invention is particularly adapted to be utilized as a secondary test in devices incorporating a suitable primary test such as that utilizing the grid detection technique and apparatus. While the grid detection test has, in the past, been adequate for determining the validity of an instrument purported to be valid, the advent of sophisticated photocopy machines, especially those capable of producing a double-sided copy, has necessitated the utilization of secondary tests for the validity determination. Indeed, many photocopies today are capable of discerning and copying the grid networks generally tested in the primary test.
It has been found that many securities have unique color arrangements upon them and that certain areas thereon are printed with an ink which is either absorptive or transparent to infrared light. Presently known photocopy machines are generally incapable of reproducing colored photocopies and are further incapable of duplicating those areas which are transparent or absorptive to particular types of light such that the reproduced areas exhibit such absorptive or transparent qualities. Consequently, a secondary test to determine the presence of particular colored areas and ink characteristics upon a paper purported to be a valid security provides a means for determining whether or not a paper passing the primary or grid test is indeed a valid instrument of merely a photocopy thereof.